1. Field of the Invention
The present invention relates to a radio frequency quadrupole linac (hereinafter referred to as an RFQ linac) of a variable energy type, for efficiently accelerating low-energy charged beams, which is utilized, for example, as an ion implantation apparatus and so forth.
2. Description of the Related Art
FIG. 1A is a partially cut perspective view showing a simplified conventional split coaxial RFQ linac, which was shown in, for example, "Structure and RF Characteristics of the INS 25.5-MHz Split Coaxial RFQ" (collected papers, from page 92 to page 94), published in the "Proceedings of the 7th Symposium on Accelerator Science and Technology which was held on Dec. 12 to 14, 1989. In the FIG., 1 is a cylindrical cavity, 2 is waving electrodes as electrodes for generating radio frequency quadruple electric field for focusing and accelerating charged particles, and 3 is a back plate for reinforcing the waving electrodes 2. 4 is stems used for shortening each pair of the opposing waving electrodes 2 to the cylindrical cavity 1, and 5 is ports for vacuum exhaust, for inputting a radio frequency, and for mounting a frequency tuner and so forth. The cylindrical cavity 1 attached with the waving electrodes 2 is referred to as an accelerating cavity. These elements are all made of conductor.
FIG. 1B is a perspective view expressing an expanded view of only the waving electrodes 2 shown in FIG. 1A. FIG. 1C is a cross-sectional view in the beam axis direction of the RFQ linac shown in FIG. 1A, and FIG. 1D and FIG. 1E are its A-A' cross-sectional view indicated by arrows and its B-B' cross-sectional view indicated by arrows. Note that, in FIG. 1A and FIGS. 1D and 1E, current paths and magnetic field paths are shown together, and the back plate 3 is omitted.
Further, FIGS. 2A and 2B are cross-sectional views showing a reentrant type cavity. In the FIG., 6 is an inner conductor. FIGS. 3A and 3B are cross-sectional views showing a reentrant type cavity in which the shape of the inner conductor 6 is modified, and FIGS. 4A and 4B are cross-sectional views of a 4-electrode cavity similar to the split coaxial cavity shown in FIG. 1C by adding a further pair of the inner conductors shown in FIGS. 3A and 3B.
FIG. 5 to FIG. 8 are perspective views or cross-sectional views showing an electrode part or an accelerating cavity of different type RFQ linac. FIG. 5 is the perspective view showing an outline of angular type electrodes. In the FIG., 7 is angular-type electrodes (here, they are so referred), that have a structure in which two metal rods having sharpened tops are mounted on a ring-shaped conductor. The angular-type electrodes 7 are mounted instead of the waving electrodes 2 in FIG. 1A. The accelerating cavity provided with the angular type electrodes 7 is referred to as an angular electrode type RFQ linac. FIG. 6A is the perspective view showing the outline of a 4-rod electrode type RFQ linac, and FIG. 6B is the expanded perspective view showing the shape of the rod electrodes. In the FIG., 8 is the rod electrodes which are supported by the stems 4 to be assembled in the cylindrical cavity 1 as shown in FIG. 6A. FIG. 7A is its partially cut front view showing a 4 vane type RFQ linac, and FIG. 7B is a cross-sectional view thereof. In the figures, 9 is vanes the top parts of which have the same configurations as the waving electrodes 2, 10 is a loop coupler (radio frequency system) for inputting a radio frequency, 11 is side tuners, and 12 is tuner driving mechanism. FIG. 8 is a cross-sectional view of a Double-H type RFQ linac which has such a structure as the one provided with two open pipes instead of the vanes 9 in FIG. 7A and FIG. 7B, and, as the electrodes, the rod electrodes 8 and so forth are mounted.
Next, the operation will be described. The RFQ linac is the one for focusing and accelerating charged beams by a radio frequency electric field, generally used in the initial stage of a high-energy accelerator for a nuclear test and so forth. Since the RFQ linac is the one for focusing and accelerating charged particles by a radio frequency quadrupole electric field, the shape of the electrodes or the structure of the accelerating cavity is not limited to one. Those shown in FIG. 1A and FIG. 5 to FIG. 8 are typical ones, however, since apparatus relating to the split coaxial RFQ linac shown in FIG. 1A to 1E will be typically described as embodiments of the present invention, the split coaxial RFQ linac shown in FIG. 1A to 1E will be described here in detail.
The waving electrodes 2 in FIG. 1B have waving shapes in the beam axis direction (this waving is referred to as modulation). By applying alternating voltages of the same sign to the opposing pair of the waving electrodes 2, and by applying alternating voltages of the reverse sign to the other pair of the electrodes 2, a quadrupole electric field is generated in the aperture surrounded by the four waving electrodes 2 through which the charged beams are passed. By the quadrupole electric field, a focusing force is applied to the charged beam, and, in addition, since an electric field is generated by the waving shape in the beam advancing direction, the beam is accelerated by this component. Since the waving period must be the value proportional to the product of the wavelength of the alternating voltage and the beam speed (the reason for this will be described later), the waving period is formed to be longer in accordance with the acceleration of the charged beams. Therefore, once the waving electrodes 2 are fabricated, the speed of the charged beams is determined by the frequency of the alternating voltage. Namely, as long as the frequency is not changed, the energy of the beam emitted from the accelerating cavity is not changed. To apply the alternating voltage to the waving electrodes 2, a method is employed in which a radio frequency power is supplied into the accelerating cavity to establish a standing wave (resonance state). This method can efficiently supply the power.
In the following, in the split coaxial cavity, why the voltage as mentioned above develops at the four waving electrodes, and what kind of electromagnetic field is generated, will be described. FIGS. 2A and 2B show a reentrant-type cavity generally used to accelerate charged particles. In this reentrant type cavity, the inner conductor 6 in the coaxial resonant cavity is cut and separated at its center to generate a concentrated electric field in the gap therebetween so that the particles are accelerated by the electric field. The distributions of the electric field and the magnetic field, and the paths of the surface currents are shown in the figure. The potential difference between the cut and separated inner conductors 6 is uniform across the cross section of the cylinder of the inner conductors 6.
FIG. 3A and FIG. 3B show a modification of the above-mentioned reentrant type cavity in which the region of the strong electric field is expanded. The distribution of the electromagnetic field and the current paths are the same as those in the reentrant type shown in FIG. 2A and 2B, and the potential difference between the inner conductors 6 is also constant. FIG. 4A and 4B show a structure in which a pair of the inner conductors is further added, and the waving electrodes 2 are employed instead of the inner conductors 6. By connecting three cavities of this type, an equivalence to the RFQ linac shown in FIG. 1A to FIG. 1E is realized. The electromagnetic field and the current paths are shown in FIG. 1A to 1E, however, the voltage between the waving electrodes is the voltage necessary to generate the already explained quadrupole electric field, and, in addition, is constant in the beam advancing direction. The back plate 3 in FIG. 4A is the one for mechanically reinforce the waving electrodes 2.
In FIG. 1C to 1E, the four waving electrodes 2 are one body or are originally fabricated separately and then combined. In either case, they are one body in the radio frequency sense. Accordingly, by supplying a radio frequency power from an arbitral position of the accelerating cavity, a predetermined potential distribution can be obtained over the whole of the waving electrodes 2. The reason why the connecting surfaces between the electrodes are formed such as A-A' cross section indicated by arrows and B-B' cross section indicated by arrows is that only one vacuum exhausting unit is necessary when the cavity is to be made vacuous, that the radio frequency power is easily transmitted throughout the cavity when the radio frequency power is supplied from one portion, and so forth. Even when the portion of the connecting surface other than the portion where the beam passes is completely covered with something, the radio frequency power is transmitted to the other cavity even when the radio frequency power is supplied from one point as long as the waving electrodes are connected. In addition, even when the connecting surfaces, namely, the stems 4, are removed, the voltage distribution between the waving electrodes is the same as the one in the above-mentioned structure, however, in this case, since the electrodes are supported at only one side, if the electrodes are long, they become mechanically instable and therefore are not practical. Generally, to stably fix the electrodes, the back plate 3, which is omitted from the illustration in FIG. 1C to FIG. 1E, is attached to the electrodes.
To efficiently supply a radio frequency power to the such an accelerating cavity as mentioned above, the frequency of the radio frequency power must coincide with the resonant frequency of the accelerating cavity. The resonant frequency in a well-known electric circuit is determined by a product of a capacitance C and an inductance L connected in parallel, and is given by the following expression. ##EQU1##
In the case of this accelerating cavity, the capacitance C is given as the sum of a capacitance C.sub.vv between the waving electrodes 2 and the capacitance C.sub.vs between the waving electrodes 2 and the stems 4. Also, the inductance L is given from L.sub.s obtained from the magnetic field generated to surround the waving electrodes 2 and L.sub.s obtained from the magnetic field surrounding the stem 4, as the following expressions. ##EQU2##
Here, l.sub.m is the length of the interval separated by the stems 4, r.sub.c is the inner radius of the cylindrical cavity 1, and r.sub.E is the effective radius of each of the waving electrodes 2. Accordingly, when the frequency of the radio frequency power is previously determined, the gap length between the electrodes and the cross sectional area of the cylindrical cavity 1 must be determined in such a way that the resonant frequency in the accelerating cavity become the same as the radio frequency of the power. Generally, the gap between the electrodes is so determined as to be able to generate a high electric field by a voltage as low as possible. Therefore, the cross section of the cylindrical cavity is determined in such a way that the necessary resonant frequency can be obtained by a capacitance C which is determined by the gap. In the practical fabrication, however, since a fabrication error is always produced, the resonant frequency is slightly shifted. To correct this, generally, a tuner having a metal block to be inserted into and to be withdrawn from the cylindrical cavity 1 is provided, and, by inserting or withdrawing it, the inductance L.sub.T is equivalently changed so that the resonant frequency is finely adjusted. In such an adjusting method, the changed spread of the resonant frequency with respect to the resonant frequency is about 1%.
As the electrodes, other than the waving electrodes 2, there is a case in which the angular electrodes 7 shown in FIG. 5 are employed. In this case, the structure is such that the electrodes of two metal rods having sharpened tops and mounted on ring-shaped conductors are mounted on the upper and lower or right and left back plates 3. As shown in FIG. 5, the angular electrodes; are alternatingly mounted in such a way that certain angular electrodes are mounted on the upper and lower back plates 3, and the next angular electrodes are mounted on the right and left back plate 3, thereby the quadrupole electric field is generated between the angular electrodes 7.
Also, by providing the electrodes as shown in FIGS. 6A and 6B, in the cylindrical cavity 1, a similar quadrupole electric field can be obtained. The potential distribution for this is considered by assuming that the two stems 4 and the two rod electrodes 8 associated therewith are one set. When the roots (opposite to the rod electrodes) of the stems 4 are assumed to be the earth potential, a structure equivalent to the well known coaxial resonator is obtained so that a distribution can be obtained in which the voltage at the root of each stem 4 is zero and the voltage at the center of each rod electrode 8 is maximum. Also, since the capacitance at the rod electrode 8 is very large, the change of the phase at the rod electrode 8 is small so that the potential difference between the rod electrodes 8 is almost constant in the space between the stems 4. Note that FIG. 6B is an expanded perspective view showing only the rod electrodes 8.
Other than those, there is a 4-vane type as shown in FIG. 7A and FIG. 7B. This is the one in which the four vanes 9 each having the top having the same structure as the waving electrode 2 are mounted in the cylindrical cavity 1. Note that, at the both end portions of the cylindrical cavity 1, there are provided spaces, and by the provision of the spaces, it is possible to generate a magnetic field which surrounds the vanes. Thereby, the top portions of the four vanes 9 function as electrodes to focus and accelerate the charged particles. At this time, if the capacitance between the vanes 9 is constant along the beam axis, the voltage between the vanes 9 is also constant. The side tuner 11 in FIG. 7B is provided for the electric field distribution adjustment and the resonant frequency adjustment, and have the same structure as the resonant frequency adjusting tuner previously mentioned a little in the split coaxial RFQ linac. Also in FIG. 7A and FIG. 7B, a loop coupler 10 for supplying a radio frequency power is depicted.
FIG. 8 shows the Double-H type which has a structure including two cleaved pipes instead of the vanes 9, and the rod electrodes 8 shown in FIG. 6B for example are fixed at the cleaved portions.
Now, as described above, the energy of the emitted beam can be varied by changing the resonant frequency (operating frequency). An example of the conventional RFQ linac according to this method will be described in the following.
FIG. 9A is a diagram showing an outline of a conventional variable energy type RFQ linac disclosed in "Acceleration Experiments of a Variable Energy RFQ Driven by an LC-tank Circuit" (collection of papers from page 95 to page 97) published in "Proceedings of the 7th Symposium on Accelerator Science and Technology) held on Dec. 12 to 14, 1989, and FIG. 9B is its equivalent circuit diagram. In the figure, 13 is a tank-type inductance, 14 is a variable capacitance, and 15 is a radio frequency power supply.
In the variable energy type RFQ linac type, the accelerating cavity as a whole is not a resonator, but a resonant circuit is formed by connecting, in parallel with the electrodes, the variable capacitance 14 of lumped constant and the tank-type inductance 13. Since the capacitance 14 of the lumped constant is employed, the power efficiency is bad but there is an advantage in that the resonant frequency can be easily and largely changed.
On the other hand, when a beam passing efficiency is assumed to offer no problem, beams with various energies can be obtained by lowering the radio frequency power in a single accelerating cavity. First, a beam accelerating method in the RFQ linac will be described. Not only the RFQ linac but also any linac for acceleration by a radio frequency power has a periodic structure consisting of a plurality of cells. The length of each cell is equal to the distance in which the phase of the radio frequency power is changed by .pi. or 2.pi.. The charged particles are accelerated in all of the cells. Accordingly, the cell length is elongated in accordance with the increase of the speed of the particles due to the acceleration. Generally, the radio frequency phase (synchronized phase .phi..sub.s) when a particle passes through the center of each cell is designed to be always constant. Namely, the electrodes are designed in such a way that the change of the phase of the radio frequency power when the particle advances from the center of one cell to the center of the next cell is always 2 .pi. (for the case of the RFQ linac, it is .pi. even though it depends on the type of the accelerating cavity).
The above explanation is for the synchronized particles, however, since the incident beams have limited lengths even when they are previously bunched, the phases of the radio frequency power at the center of each cell are different depending on the particles so that the increases in energy are different. Therefore, only the synchronized particles are accelerated in accordance with the design, but the energies of the asynchronized particles are gradually shifted from the designed values.
If, however, the accelerating phase (.phi..sub.s) of the radio frequency for the synchronized particles is set between -90 degree and 0 degree of the cosine wave, the asynchronized particles other than the synchronized particles are accelerated with vibrations in the sense of energy and phase around the synchronized particles. The orbit drawn on a phase-energy plane by the outer-most particle is called as a separatrics. The particles outer thereof do not oscillate in phase so that, along with the advance of the particles, there are positions through where they pass with phases of decreasing speeds. As a result, the particles outer of the outer-most particle are emitted without being accelerated. The separatrics is maximum when .phi..sub.s =90 degrees (but the particles are not accelerated when averaged), and is disappeared when .phi..sub.s =0 degrees. Namely, when .phi..sub.s =0 degrees, the synchronized particles are accelerated most efficiently, and the particles around them repeat to be accelerated and decelerated so that the average accelerating voltage becomes zero.
Even when the electrode voltages are changed by changing the radio frequency power inputted into the accelerating cavity, the speeds (or energy) of the synchronized particles are not changed. This will be described in the following. The increment of the energy of a particle in each cell is expressed as the following expression. EQU .DELTA.W.varies.V.sub.0 COS .phi..sub.s
Here, V.sub.0 is a voltage between the electrodes, and T is a coefficient taking into account the electric field distribution in each cell and the change of the radio frequency phase when the particle passes through the cell. Each cell length is determined based on the energy increase obtained from the above expression, thereby the electrodes are designed. Accordingly, the synchronized particle is the particle which obtains the designed value .DELTA.W when it passes through one cell. Therefore, even when V.sub.0 is changed, the synchronized particle is accelerated in such a way that the above expression .DELTA.W is constant, so that .phi..sub.s changes depending on the change of V.sub.0. Here, the change of the coefficient T is neglected. Namely, a particle incident at a time of a changed .phi..sub.s becomes a synchronized particle, and the original synchronized particle becomes an asynchronous particle to be accelerated with vibration around the new synchronized particle in the sense of energy.
When the electrode voltage is raised, .phi..sub.s infinitely closes with 90 degrees. By contrast, when the electrode voltage is lowered, since the separatrics disappears at .phi..sub.s =0 degrees, the situation in which the particle is not accelerated is not changed even when the voltage is lowered below the voltage at which .phi..sub.s =0 degrees. Note that the acceleration is not effected as a result of repeating accelerations and decelerations. Therefore, this case is applied to a linac in which several times or more of the phase vibrations are carried out. In a linac in which the phase vibration is about one or less, the energy spread of the emitted beam is large but the central energy is changed.
Next, a description will be given with respect to the RFQ linac. In the RFQ linac, generally, continuous beams are inputted, and are bunched along with an acceleration (.phi..sub.s is at first 90 degrees and is gradually approached to the final value), and, after .phi..sub.s reaches the final value, they are accelerated under the condition in which .phi..sub.s is constant. The region where .phi..sub.s is constant is referred to as an accelerated portion. According to this method, a more number of particles can be accelerated. The state at this time is shown in FIG. 10. In the figure, the abscissa represents a phase of the radio frequency, and the ordinate represents the energy (Ein: incident energy, Eex: emitting energy). In the left side of the figure, a normal acceleration is shown from which it will be seen that continuous beams are bunched along with an acceleration. In addition, due to the phase vibration, the bunched beams have a certain energy width.
It has already been described that .phi..sub.s is changed in accordance with the change of the accelerating voltage from the designed value. When the accelerating voltage is raised, .phi..sub.s changes to the direction of -90 degrees, and when it is lowered, .phi..sub.s changes to the direction of 0 degrees. Namely, when the accelerating voltage becomes too low, the separatrics disappears in a partial region, for example, in the accelerated portion. In the right side in FIG. 10, the state of the disappear of the separatrics when the accelerating voltage is lowered by 20% is shown. Accordingly, by designing the RFQ linac in such a way that the phase vibration in the accelerated portion is one time or less, the energy spread is expanded so that it is sufficient to use only the particles emitted with the necessary energy. In this case, a number of particles are forced out from the separatrics until they reach the accelerated portion so that the energy of the emitted beams are extremely expanded, resulting in that the number of particles within a unit energy width becomes extremely small.
Since the conventional RFQ linac is constructed as above, it is difficult to largely change the speed, i.e., energy, of the emitted beam. Even when it can be changed, there is a problem in that the accelerating cavity has a bad power efficiency, or the beam current per unit energy width is extremely small. When it is used in a portion where variable energy is not necessary such as in an initial stage of a high energy accelerator for a nuclear test, that is not a particular problem. When it is used in a portion where the energy is required to be largely varied for the same charged particles, such as an ion implanting apparatus and so forth for example, however, there is a problem in that such an RFQ linac cannot be used alone.